Mg-ta based dielectric ceramic for multi-layer ceramic capacitor and low-temperature preparation method thereof

ABSTRACT

A Mg—Ta based dielectric ceramic for multi-layer ceramic capacitor (MLCC) and a low-temperature preparation method thereof are provided. By providing a glass additive with high matching with a Mg—Ta ceramic, a modifier A+12CO3—B2+O—C3+2O3—SiO2 (A=Li, K; B=MnO, CuO, BaO; C=B, Al) is intruded in to a main material MgO—Ta2O5, which can significantly reduce the sintering temperature and provide a negative temperature coefficient of dielectric constant of −100±30 ppm/° C., and reduce the deterioration factors of loss caused by an additive for sintering, and prepare a dielectric material applied to RF MLCC with low loss, low cost and good process stability.

TECHNICAL FIELD

The disclosure relates to the field of electronic ceramics andmanufacture thereof, and more particularly to a magnesium-tantalum(Mg—Ta) based dielectric ceramic for multi-layer ceramic capacitor formulti-layer ceramic capacitor (MLCC) and a low-temperature preparationmethod thereof.

BACKGROUND

With the rapid development of mobile communication, miniaturization andintegration of microelectronic devices have become the mainstream. TheMLCC plays an irreplaceable role in military, electronic information,mobile communication, aerospace and other fields because of itsadvantages such as large capacity, small volume and suitability for massproduction. Especially driven by the 5-th generation (5G) technology,the requirements for the MLCC will grow steadily in the future. The MLCCincludes three parts: an internal electrode, a terminal electrode and aceramic medium, in which the internal electrode (such as silver (Ag),Ag/platinum (Pt)) and the ceramic medium are parallel to each other toform a main body, and the terminal electrode generally has a three-layerstructure: an innermost layer is Ag or Ag—Pt, which plays a link roleand leads out the internal electrode; a middle layer is a blockinglayer, usually is nickel (Ni) or Cuprum (Cu), which mainly prevents Agfrom being corroded by molten solder during welding; and an outermostlayer is a welding layer. The classification of ceramic capacitors isoften expressed by a temperature coefficient τ_(ε) of a dielectricconstant of the used dielectric ceramic. According to the RS-198standard of electronic industry association (EIA) in the United Statedof America (U.S.A) (EIA RS-198), the dielectric ceramics can usually bedivided into three classes according to their temperature stability, theclass I ceramic, the class II ceramic and the class III ceramic.Specifically, the class I ceramic capacitors have high stability and lowloss, the class II ceramic capacitors have high volumetric efficiencybut poor stability, and are suitable for buffering, decoupling andbypassing circuits, the class III ceramic capacitors have highervolumetric efficiency but poorer stability. Due to high stability andlow loss, Class I ceramic capacitors are most widely used in radiofrequency (RF) and microwave communication. Naming rules vary accordingto the EIA RS-198. For example, ceramic capacitors with M2G temperaturecharacteristics refer to a temperature drift of −100±30 ppm/° C. in atemperature range of −55° C. to 85° C., which can be used to preparetransmitter and receiver (T/R) components of phased-array radars, RFpower amplifiers, transmitters and other circuits for coupling,coordination and filtering.

A traditional dielectric material for the MLCC is based on TiO₂, and asintering temperature, dielectric properties, and a temperaturecoefficient of a dielectric constant are adjusted by ion substitution orcomposite doping. For example, in a barium-titanium (Ba—Ti) system,BaTiO₃ is the earliest representative of the Class II ceramic capacitorsfor commercial application, while BaTi₄O₉ is a typical reprehensive ofthe Class I ceramic capacitors. A Mg—Ti system, represented by MgTiO₃with excellent dielectric properties, is a potential low-loss dielectricmaterial in radio frequency communication, but its further practicalapplication is limited due to the high sintering temperature (greaterthan 1400° C.).

A Mg—Ta based dielectric ceramic is a kind of material with a moderatedielectric constant and an extremely low dielectric loss (such asMgTa₂O₆: ε_(r) is 25, Q×f is 131000 GHz, and tan δ is 7×10⁻⁵), which ispotential dielectric material for the class I ceramic capacitors.However, the sintering temperature of this ceramic is too high (greaterthan 1400° C.), although some researchers have tried to reduce thesintering temperature, for example, when 0.5 wt. % B₂O₃ is added, asintering temperature of MgO—Ta₂O₅ ceramic is 1550° C. (ε_(r) is 27.8,Q×f is 180000 GHz); When 0.4 wt. % MgO is added, the sinteringtemperature of MgO—Ta₂O₅ ceramic is still high at 1425° C. (ε_(r) is25.3, Q×f is 160000 GHz); when a sol-gel method is used, the powder isuniform at nanometer level, and the sintering temperature is moderate(1200° C.) but the dielectric loss is increased (ε_(r) is 30.1, Q×f is57300 GHz), and the sol-gel method is complex and not suitable for massproduction; and when 5 wt. % CuO is added, the sintering temperature isobviously improved (as low as 1100° C.), but the Q×f value is obviouslydeteriorated (as low as 21200 GHz).

Based on the above research status, it is not difficult to find that theMgTa₂O₆-based ceramic in the related art cannot maintain excellentdielectric properties at a lower sintering temperature. Therefore, it isnecessary to, according to the application requirements of RF ceramiccapacitors with current M2G temperature characteristics, develop atemperature-compensated RF MLCC material with simple and controllableprocess, low dielectric loss, co-firing with an Ag70/Pd30 internalelectrode (1080° C. to 1150° C.) and a stable temperature coefficient ofdielectric constant, to meet the application requirements of an RFcommunication industry.

SUMMARY

The purpose of the disclosure is to provide a Mg—Ta based dielectricceramic for multi-layer ceramic capacitor and a low-temperaturepreparation method thereof, thereby to overcome the technical problemsof the MgTa₂O₆-based ceramic that cannot balance the low sinteringtemperature and the excellent dielectric properties.

In order to achieve the above purpose, the disclosure adopts thefollowing technical solutions.

A Mg—Ta based dielectric ceramic for multi-layer ceramic capacitor isprovided, which including a ceramic material and a modifier;

-   -   a chemical formula of the ceramic material is MgTa₂O₆;    -   a formula of the modifier is A⁺¹ ₂CO₃—B²⁺O—C³⁺ ₂O₃—SiO₂ with a        mass ratio of 31:17:36:16 (wt. %), A⁺¹ ₂CO₃ consists of 20 wt. %        of Li₂CO₃ and 11 wt. % of K₂CO₃; B²⁺O— consists of 2 wt. % BaO,        5 wt. % of MnO and 10 wt. % of CuO; and C³⁺ ₂O₃ consists of 30        wt. % of B₂O₃ and 6 wt. % of Al₂O₃;    -   where a formula of the Mg—Ta based dielectric ceramic is        MgTa₂O₆+x wt. % of the modifier, a value of x of the modifier is        in a range of 1 to 2, the Mg—Ta based dielectric ceramic is        prepared by a solid-state method, a main crystal phase of the        Mg—Ta based dielectric ceramic is MgTa₂O₆ phase with a Trirutile        structure, a sintering temperature of the Mg—Ta based dielectric        ceramic is in a range of 1050° C. to 1150° C., a dielectric        constant of the Mg—Ta based dielectric ceramic is in a range of        20 to 28, a dielectric loss of the Mg—Ta based dielectric        ceramic is in a range of 1.7×10⁻⁴ to 5.0×10⁻⁴, a value of a        quality factor Q×f is in a range of 16000 GHz to 45000 GHz, and        a temperature coefficient of the dielectric constant is stable        and meets M2G temperature characteristics (−55° C.: −118 (parts        per million)ppm/degree Celsius (° C.); 85° C.: −110 ppm/° C.).

In an embodiment, in a situation that the value of x is 2, under thesintering temperature of 1150° C., the dielectric constant of the Mg—Tabased dielectric ceramic is 26.87, the dielectric loss is 1.71×10⁻⁴, andthe value of the quality factor Q×f is 44398 GHz.

A method for preparing the above preparing the Mg—Ta based dielectricceramic for multi-layer ceramic capacitor is provided, which includesthe following steps:

-   -   step 1, mixing raw powders of magnesium oxide (MgO) and Ta₂O₅        according to the chemical formula MgTa₂O₆ to obtain a mixed        powder;    -   step 2, putting the mixed powder prepared in step 1 into a ball        milling tank, using zirconium balls and deionized water as a        grinding medium and performing planet ball milling for 4-6 hours        according to a mass ratio of the mixed powder:the zirconium        balls:the deionized water of 1:4-5:2-4 to obtain a mixed slurry,        drying the mixed slurry in an oven after the planet ball milling        and then sieving with a 60-100 mesh sieve to obtain a sieved        powder; and sintering the sieved powder in an atmosphere of        900-1000° C. for 3-5 hours to obtain MgTa₂O₆;    -   step 3, mixing raw powders of Li₂CO₃, K₂CO₃, BaO, MnO, CuO,        B₂O₃, Al₂O₃, and SiO₂ according a mass ratio of        Li₂CO₃:K₂CO₃:BaO:MnO:CuO:Al₂O₃:SiO₂ of 20:11:2:5:10:30:6:16 to        obtain a second mixed powder; using the zirconium balls and        alcohol as a grinding medium and performing planet ball milling        for 6-8 hours according to a mass ratio of the second mixed        powder:the zirconium balls:the alcohol of 1:4-5:4-6 to obtain a        ball-milled material, sintering the ball-milled material at a        temperature of 600-650° C. for 3-6 hours after drying the        ball-milled material and then heating to a temperature of        1450-1550° C. for melting for 3-5 hours to obtain a melted        material, pouring the melted material into the deionized water        for cooling to obtain a cooled material, and grinding the cooled        material to a uniform power as the modifier;    -   step 4, mixing the modifier prepared in step 3 into MgTa₂O₆        prepared in step 2 according to the formula of the Mg—Ta based        dielectric ceramic MgTa₂O₆+x wt. % to obtain a third mixed        power, x is in a range of 1 to 2, performing planetary ball        milling for 3-5 hours according to a mass ratio of the third        mixed powder:the zirconium balls:the deionized water of        1:4-5:3-5 to obtain a second ball-milled material, and adding a        polyvinyl alcohol solution, as a binder, into the second        ball-milled material after drying the second ball-milled        material to perform granulation to obtain a ceramic raw        material; and    -   step 5: performing press-molding on the ceramic raw material        prepared in step 4, heating at a heating rate of 2-5° C./min and        discharging glue at 600-650° C. for 3-5 hours, and then heating        at the same heating rate to a temperature of 1050-1150° C. and        keeping the temperature for 4-6 hours, thereby obtaining a        modified MgTa₂O₆ dielectric ceramic material.

In the field of low-temperature sintered ceramics, the driving force oflow-temperature sintering comes from the matching relationship between aglass additive (also referred as to modifier) and a ceramic, that is,the glass additive produces a liquid phase when a temperature is above amelting point. If they have good matching, a ceramic powder can bewetted in the liquid phase, which leads to dissolution and completes amass transfer mechanism of “dissolution-precipitation” to realizelow-temperature densification. However, the matching between the glassadditive and the ceramic is unique, so in the disclosure, a modifier A⁺¹₂CO₃—B²⁺O—C³⁺ ₂O₃—SiO₂ (A=Li, K; B=MnO, CuO, BaO; C=B, Al) is intrudedin to the main material MgO—Ta₂O₅, which can significantly reduce thesintering temperature and provide a negative temperature coefficient ofdielectric constant of −100±30 ppm/° C., and reduce the deteriorationfactors of loss caused by an additive for sintering, and prepare adielectric material applied to RF MLCC with low loss, low cost and goodprocess stability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic diagram of an X-ray diffraction (XRD)pattern according to an embodiment 6 of the disclosure.

FIG. 2 illustrates a schematic diagram of scanning electron microscope(SEM) according to the embodiment 6 of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

The disclosure will be further described in detail with the attacheddrawings and embodiments.

A method for preparing a Mg—Ta based dielectric ceramic for multi-layerceramic capacitor is provided and includes the following steps: step 1,mixing raw powders of magnesium oxide (MgO) and Ta₂O₅ according to thechemical formula MgTa₂O₆ to obtain a mixed powder;

-   -   step 2, putting the mixed powder prepared in step 1 into a ball        milling tank, using zirconium balls and deionized water as a        grinding medium and performing planet ball milling for 6 hours        according to a mass ratio of the mixed powder:the zirconium        balls:the deionized water of 1:5:4 to obtain a mixed slurry,        drying the mixed slurry in an oven after the planet ball milling        and then sieving with a 100 mesh sieve to obtain a sieved        powder; and sintering the sieved powder in an atmosphere of        1000° C. for 5 hours to obtain MgTa₂O₆;    -   step 3, mixing raw powders of Li₂CO₃, K₂CO₃ BaO, MnO, CuO, B₂O₃,        Al₂O₃, and SiO₂ according a mass ratio of        Li₂CO₃:K₂CO₃:BaO:MnO:CuO:B₂O₃:Al₂O₃:SiO₂ of 20:11:2:5:10:30:6:16        to obtain a second mixed powder; using the zirconium balls and        alcohol as a grinding medium and performing planet ball milling        for 6 hours according to a mass ratio of the second mixed        powder:the zirconium balls:the alcohol of 1:5:4 to obtain a        ball-milled material, sintering the ball-milled material at a        temperature of 600° C. for 5 hours after drying the ball-milled        material and then heating to a temperature of 1500° C. for        melting for 4 hours to obtain a melted material, pouring the        melted material into the deionized water for cooling to obtain a        cooled material, and grinding the cooled material to a uniform        power as the modifier;    -   step 4, mixing the modifier (also referred to as glass additive)        prepared in step 3 into MgTa₂O₆ (also preferred to as pre-fired        material) prepared in step 2 according to the formula of the        Mg—Ta based dielectric ceramic MgTa₂O₆+x wt. % to obtain a third        mixed power, x is 1 or 2, performing planetary ball milling for        5 hours according to a mass ratio of the third mixed powder:the        zirconium balls:the deionized water of 1:5:4 to obtain a second        ball-milled material, and adding an 8 wt. % of polyvinyl alcohol        solution, as a binder, into the second ball-milled material        after drying the second ball-milled material to perform        granulation to obtain a ceramic raw material; and    -   step 5: performing press-molding on the ceramic raw material        prepared in step 4, heating at a heating rate of 5° C./min and        discharging glue at 650° C. for 4 hours, and then heating at the        same heating rate to a temperature of 1050-1150° C. and keeping        the temperature for 6 hours, thereby obtaining a modified        MgTa₂O₆ dielectric ceramic material.

In order to better illustrate the effect of the disclosure, sixembodiment samples are made according to the above steps. FIG. 1 is theXRD pattern of an embodiment 6. After searching, the phase compositionof the ceramic corresponds to the standard card of MgTa₂O₆, i.e. jointcommittee on powder diffraction standards (JCPDS) card with No. 32-0631,no second phase diffraction peak is found in the system at this time,which indicates that ion substitution will not change the crystalstructure at this doping amount, and this type of ceramic belongs to theMgTa₂O₆ structure.

FIG. 2 is a SEM topography diagram of the embodiment 6. As can be seenfrom FIG. 2 , the grain size is small, and there are fewer pores.

The composition and microwave dielectric properties of the embodimentsare shown in Table 1 and Table 2 as follows.

Table 1 shows the components of each embodiment sample.

Embodiment number 1 2 3 4 5 6 Mass of MgO 8.358 8.358 8.358 8.358 8.3588.358 each Ta₂O₅ 91.642 91.642 91.642 91.642 91.642 91.642 componentLi₂CO₃ 0.200 0.200 0.200 0.400 0.400 0.400 K₂CO₃ 0.110 0.110 0.110 0.2200.220 0.220 BaO 0.020 0.020 0.020 0.040 0.040 0.040 MnO 0.050 0.0500.050 0.100 0.100 0.100 CuO 0.100 0.100 0.100 0.200 0.200 0.200 B₂O₃0.300 0.300 0.300 0.600 0.600 0.600 Al₂O₃ 0.060 0.060 0.060 0.120 0.1200.120 SiO₂ 0.160 0.160 0.160 0.320 0.320 0.320 Sintering temperature1050 1100 1150 1050 1100 1150 (° C.)

Table 2 shows the dielectric properties of each embodiment sample.

Embodiment dielectric tanδ Q × f τ_(∈) (ppm/° C.) number constant ∈_(r)(10⁻⁴) (GHz) −55° C. 85° C. 1 21.33 3.09 25793 −94 −98 2 24.69 2.2533956 −107 −115 3 27.81 1.88 40521 −114 −105 4 20.38 4.89 16587 −80 −885 25.17 1.82 42665 −103 −112 6 26.87 1.71 44398 −118 −110

From the data shown in Table 1 and Table 2, it can be seen that in theembodiment 6, when the sintering temperature is 1150° C., the dielectricconstant and Q×f value of the modified MgTa₂O₆ dielectric ceramicmaterial get the best values: ε_(r)=26.87, tan δ=1.71×10⁻⁴, Q×f=44398GHz, and τ_(ε) is in a range of −118 ppm/° C. to −110 ppm/° C. Comparedwith literature reports in the related art, the sintering temperature isgreatly reduced and the dielectric loss is kept low, at the same time,the temperature coefficient of dielectric constant is relatively stablein the range of −55° C. to 85° C., the modified dielectric ceramicmaterial is suitable for industrial application.

What is claimed is:
 1. A magnesium-tantalum (Mg—Ta) based dielectricceramic for multi-layer ceramic capacitor, comprising a ceramic materialand a modifier; wherein a chemical formula of the ceramic material isMgTa₂O₆; a formula of the modifier is A₂CO₃—BO—C₂O₃—SiO₂ with a massratio of 31:17:36:16, A₂CO₃ consists of 20 wt. % of Li₂CO₃ and 11 wt. %of K₂CO₃; BO consists of 2 wt. % BaO, 5 wt. % of MnO and 10 wt. % ofCuO; and C₂O₃ consists of 30 wt. % of B₂O₃ and 6 wt. % of Al₂O₃; aformula of the Mg—Ta based dielectric ceramic is MgTa₂O₆+2 wt. % of themodifier, the Mg—Ta based dielectric ceramic is prepared by asolid-state method, a main crystal phase of the Mg—Ta based dielectricceramic is MgTa₂O₆ phase with a Trirutile structure; under a sinteringtemperature of 1150 degrees Celsius (° C.), a dielectric constant of theMg—Ta based dielectric ceramic is 26.87, a dielectric loss of the Mg—Tabased dielectric ceramic is 1.71×10⁻⁴, and a value of a quality factorQ×f of the Mg—Ta based dielectric ceramic is 44398 GHz; and atemperature coefficient τ_(ε) of the dielectric constant is stable andmeets M2G temperature characteristics of (−55° C.: τ_(ε)=−118 parts permillion (ppm)/° C.; 85° C.: τ_(ε)=−110 ppm/° C.); the Mg—Ta baseddielectric ceramic for multi-layer ceramic capacitor is prepared throughthe following steps: step 1, mixing raw powders of magnesium oxide (MgO)and Ta₂O₅ according to the chemical formula MgTa₂O₆ to obtain a mixedpowder; step 2, putting the mixed powder prepared in step 1 into a ballmilling tank, using zirconium balls and deionized water as a grindingmedium and performing planet ball milling for 4-6 hours according to amass ratio of the mixed powder:the zirconium balls:the deionized waterof 1:4-5:2-4 to obtain a mixed slurry, drying the mixed slurry in anoven after the planet ball milling and then sieving with a 60-100 meshsieve to obtain a sieved powder; and sintering the sieved powder in anatmosphere of 900-1000° C. for 3-5 hours to obtain MgTa₂O₆; step 3,mixing raw powders of Li₂CO₃, K₂CO₃, BaO, MnO, CuO, B₂O₃, Al₂O₃, andSiO₂ according a mass ratio of Li₂CO₃:K₂CO₃:BaO:MnO:CuO:B₂O₃:Al₂O₃:SiO₂of 20:11:2:5:10:30:6:16 to obtain a second mixed powder; using thezirconium balls and alcohol as a grinding medium and performing planetball milling for 6-8 hours according to a mass ratio of the second mixedpowder:the zirconium balls:the alcohol of 1:4-5:4-6 to obtain aball-milled material, sintering the ball-milled material at atemperature of 600-650° C. for 3-6 hours after drying the ball-milledmaterial and then heating to a temperature of 1450-1550° C. for meltingfor 3-5 hours to obtain a melted material, pouring the melted materialinto the deionized water for cooling to obtain a cooled material, andgrinding the cooled material to a uniform power as the modifier; step 4,mixing the modifier prepared in step 3 into MgTa₂O₆ prepared in step 2according to the formula of the Mg—Ta based dielectric ceramic MgTa₂O₆+2wt. % to obtain a third mixed power, performing planetary ball millingfor 3-5 hours according to a mass ratio of the third mixed powder:thezirconium balls:the deionized water of 1:4-5:3-5 to obtain a secondball-milled material, and adding a polyvinyl alcohol solution, as abinder, into the second ball-milled material after drying the secondball-milled material to perform granulation to obtain a granularmaterial; and step 5: performing press-molding on the granular materialprepared in step 4, heating at a heating rate of 2-5° C./minutes (min)and discharging glue at 600-650° C. for 3-5 hours, and then heating atthe same heating rate to a temperature of 1150° C. and keeping thetemperature for 4-6 hours, thereby obtaining a modified MgTa₂O₆dielectric ceramic material.